3. Learning Intentions
• Apply Newtons second Law (F=ma)
• Know and use the quantity called momentum
(p=mv)
• Express and use Newtons Second Law in terms of
momentum F=Δp/ Δt
4. Newtons First Law
• A body will remain at constant speed
unless acted on by an unbalanced force
• Forces cause changes of velocity.
v
a
t
5. Newtons Second
Law
• The acceleration of a body is proportional to the
force that causes it.
• Heavy objects are harder to accelerate than light
ones
• This is often referred to as inertia
F a
F ma
But….
Acceleration and therefore
force are vector quantities
a
F
6. Vector form of
Newton’s Second Law
• We need to ensure we
are aware of the
direction of our vectors
(Which way is +ve?)
• A ball (0.5kg) rebounds
from a wall at right
angles. The impact
takes 0.01s.
• What is the force acting
on the ball? On the
wall?
3 ms-1
2 ms-1
7. Vector form of Newton’s
Second Law – 2D
example
•
A ball of mass 0.2kg collides
with a wall at an angle of
30o. It rebounds at the same
angle and the same speed.
The impact lasts 0.05s
a) Find the average
acceleration during impact.
b) The force on the ball (and on
the wall)
3 ms-1
30o
8. Momentum
• Newtons laws were
originally expressed in
terms of quantity called
momentum
• Forces caused changes
of momentum.
• Momentum is the
product of an objects
mass and velocity
p mv
units – kgms-1
v
m
F ma
mv
F
t
p
F
t
9. Impulse
• Why do you follow
through in tennis?
• Why do you draw your
hands back when you
catch a cricket ball
• The quantity “force x
time” is known as
impulse.
p
F
t
F t p
units – Ns
1 Ns = 1 kgms-1
10. Momentum form of Newton’s
Second Law
• We need to ensure we are
aware of the direction of
our vectors (Which way is
+ve?)
• A ball (0.1kg) rebounds
from a wall at right angles.
The impact takes 0.01s.
• What is the force acting on
the ball? On the wall?
• What is the average
acceleration during impact?
2 ms-1
1.5 ms-1
11. A car of mass 1000kg, travels
East
• A car of mass 1000kg, travels East at 6ms-1
and takes 2 seconds to round a bend, which
it does without loss of speed and then
heads South.
• What average force must turn the car?
• What is the radius of the bend?
12. Momentum form of Newton’s
Second Law – 2D example
•
A ball of mass 0.25kg
collides with a wall at an
angle of 30o. It rebounds at
the same angle and the
same speed. The impact
lasts 0.05s. Find
a) The force on the ball (and on
the wall).
b) the average acceleration
during impact
4 ms-1
30o
13. Shunts
• A NASCAR coming out of a spin is
coasting out of gear at 20 ms-1 when it
goes into the path of another car travelling
in the same direction at 40 ms-1. When
they collide the bumpers lock. How fast
do they move after the collision?
20 ms-1
800 kg
40 ms-1
750 kg
14. Slams
• Two Hockey players collide
• If they hold onto each other
after the collision, how
do they move?
80 kg
10 ms-1
90 kg
12 ms-1
15. Stones
• In curling the players slide 2kg “stones” along the ice
toward a target. They score the numbers of stones they
have nearest the centre of the target. Collisions to knock
out opponents stones are part of the game.
• In this shot explore the different
scenarios using calculations to
justify any conclusions.
8 ms-1
0 ms-1
